For reducing the exhaust emissions of an internal combustion engine, three-way exhaust catalytic converters, referred to below for simplicity as catalytic converters, have proven their worth over a long time. In an internal combustion engine at working temperature, up to 98% of hydrocarbon, carbon monoxide, and oxides of nitrogen emissions are converted with systems that are available on the market. Emission behavior, nonetheless, may be inadequate during a cold start and the immediately subsequent warm-up phase of OTTO or once-through then out combustion engines, because the catalytic converter and the lambda probe have not yet reached the operating temperatures thereof (“light-off” temperatures).
One possible measure for increasing the quality of the exhaust gases consists of blowing fresh air (so-called secondary air) into the exhaust manifold close to the exhaust valves, so that when starting the internal combustion engine, the unburnt exhaust gas components resulting from a rich startup mixture are oxidized by post-combustion at temperatures of up to 600° C. Said exothermal reaction results in an increase in the exhaust gas temperature and thereby to a reduction of the warm-up time of the catalytic converter. At the same time, the unburnt exhaust gas components are reduced.
To introduce the secondary air, a so-called secondary air pump is typically used. It may include an electrically driven compressor that sucks air from the surroundings and blows said air through a secondary air line into the exhaust duct close to the exhaust valves. In the secondary air line an electrically controlled valve, also referred to as a secondary valve, is switched on. Said valve is closed outside of the cold start and warm-up of the internal combustion engine.
In the case of combustion engines with cylinders, for example 6, 8, or 12 cylinders, that are not disposed in a single row (in-line engine), the individual cylinders are associated with so-called cylinder banks. With an 8-cylinder internal combustion engine for example, a first cylinder bank is associated with cylinders 1-4 and a second cylinder bank is associated with cylinders 5-8. In this case, each cylinder bank can be supplied with air by means of a dedicated secondary air pump with associated secondary air lines or a single secondary air pump supplies both cylinder banks, wherein a line connected to the pump output branches and a dedicated secondary air line leads to each cylinder bank. In each of the secondary air lines, a secondary valve is switched on, to blow bank-selective secondary air.
Because the secondary air system is relevant to the exhaust from the internal combustion engine, it must be checked for proper functioning during operation. Legislative measures provide that not only is the secondary air system as such to be tested, e.g., globally for operability, but in the case of a system with a plurality of cylinder banks and thus a plurality of secondary air paths, each individual subsystem must be diagnosed, e.g., bank-selective fault detection must be provided. In DE 102 49 421 A1, a method for monitoring the delivery of secondary air into the exhaust gas of an internal combustion engine is described, wherein the internal combustion engine comprises at least two partly separated exhaust systems, in each of which an exhaust gas catalytic converter and upstream thereof a lambda probe are disposed in each case. A secondary air pump transports the induced secondary air first via a single secondary air line containing an electrically controlled secondary valve and that branches downstream of the secondary valve into individual secondary air lines corresponding to the number of the exhaust systems for transporting secondary air into the respective exhaust systems. A value for the entire air flow introduced into the exhaust systems is determined from the signal of at least one air flow meter. For determining the masses of air actually introduced into the individual exhaust systems, the output signals of the lambda probes in the exhaust systems are used in order to determine an unequal distribution of the masses of air delivered to the individual exhaust systems.
In WO 2007/087905 A1, a method and a device for monitoring the secondary air system in an emission control system of an internal combustion engine are described. The individual cylinders of the internal combustion engine are divided into at least two cylinder banks and each cylinder bank is associated with a separate exhaust duct. Secondary air is transported using a compression arrangement and is introduced into the respective exhaust ducts via individual secondary air lines corresponding to the exhaust ducts. The flow of the secondary air is adjusted independently in each of the individual secondary air lines using an electrically controlled throughflow control arrangement. From the signal of a secondary air flow meter, a value for the entire secondary air flow that is introduced into the exhaust ducts is determined. The throughflow control arrangements are controlled into an open position, the value for the entire secondary air flow is detected and then the throughflow control arrangements are individually controlled into a closed position at time intervals relative to each. The values for the secondary air flows occurring during this before and after the closing of the individual throughflow control arrangements are detected and said values are compared with a plurality of different threshold values. Depending on the result of the comparisons, a differentiation is made of the fault location in relation to the respective cylinder banks.
An auxiliary air delivery system from DE 10 2004 058 398 A1 delivers auxiliary air to exhaust pipes, which are each fitted to two banks of the internal combustion engine, via an auxiliary air delivery pipe with a connection segment and two branch segments that are separated from the connection segment. An air control valve is disposed in each branch segment and a pressure sensor is provided in the connection segment. An abnormality diagnosis is conducted in relation to the two air control valves that are opened at different times, wherein the two air control valves comprise a first air control valve that is opened first and a second air control valve that is opened second, based on a combination of pressure fluctuations in the connection segment detected by the pressure sensor when opening the first air control valve and a pressure fluctuation in the connection segment detected by the pressure sensor when opening the second air control valve.
A method for the diagnosis of a secondary air system from DE 10 2007 062 794 A1, claims to detect (a) a leak in an exhaust segment downstream of the secondary valve can be detected and (b) significant faults between the secondary air pump and the secondary valve. The detection strategy of said known method consists in reducing pressure fluctuations or pressure pulsations that are produced by the opening and closing of exhaust valves of the internal combustion engine concerned in the case of a leak between the internal combustion engine and the secondary valve. The pressure deviations from an average value are thereby less than for a secondary air system with no leak.
A method and a device for diagnosing the functionality of a secondary air system comprising an air pump, an air pressure sensor, and a valve are described in DE 10 2012 222 868 A1. The method comprises the sequential operation of the secondary air system in a first operating state, in a second operating state and in a third operating state. During the sequential mode, the following steps are carried out: (a) measuring the time variation of the pressure in the secondary air system with the air pressure sensor, (b) calculating the variation with time of a form factor based on the variation with time of a measured pressure signal that is output by the air pressure sensor, (c) determining the variation with time of a signal power based on the variation with time of the pressure signal, (d) modelling the variation with time of a model signal power based on at least one variable that characterizes the operating state of the internal combustion engine, (e) calculating the variation with time of a power factor based on the determined signal power and on the modelled model signal power, and (f) diagnosing the functionality of the secondary air system based on the variation with time of the form factor and the variation with time of the power factor.